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Car fuel economy

Published fuel economy figures for new cars are expected to become more reliable with the introduction of a new official test

Since September 2018 the fuel consumption of all new cars has been measured using a new, official EU test procedure called the WLTP – Worldwide Harmonised Light Vehicle Test Procedure.

The chances are you've never heard of the WLTP or the NEDC (New European Driving Cycle) – the test procedure it has replaced – but this is a significant change, and one that should result in more realistic, more achievable official fuel consumption figures for new cars.

The gap between the fuel economy you get in the real world and the official figures that car manufacturers must quote in their brochures has been getting wider and wider.

Back in 2001 the average gap was a relatively small 8% but by 2014 this had grown to as much as 40%.

Some difference is to be expected as a laboratory test can never accurately reflect the range of driving styles, road and weather conditions experienced in the real world, but the gap had become so wide that manufacturers have been accused of misleading claims.

It seems that the more economical a car is to run on paper, the greater the gap between official figures and actual fuel consumption. This means you can’t even rely on official figures as a guide to relative performance.

Official tests

New car models have to meet a range of safety and environmental ‘type approval’ tests before they can be sold.

One such test is to determine official fuel consumption and CO2 emissions

The car is ‘driven’ on a rolling road, following a defined cycle.

Car manufacturers have to state official fuel consumption figures and aren’t allowed to quote alternative figures.

The official CO2 figure is the basis for our car tax and company car tax systems and is also used to monitor progress towards fleet average CO2 emissions reduction targets.

CO2 reduction targets

European countries must meet strict targets for fleet average CO2 emissions – 130g/km from 2015 and 95g/km from 2020. Heavier cars are allowed higher emissions if offset by lighter vehicles with lower emissions.

Car manufacturers have a strong financial interest as the fines for exceeding CO2 targets can be substantial.

How the old test worked

Official testing takes place on a laboratory rolling road and, for most cars first registered before September 2018, followed a procedure based on the New European Driving Cycle (NEDC) which has two parts: an ‘urban’ cycle and an ‘extra-urban’ cycle.

Urban – a cold start followed by around 2.5 miles at an average of 12mph and briefly reaching a maximum of 31mph

Extra-urban – following straight on from the urban cycle, the car covers 4.3 miles at an average of 39mph

Three official fuel consumption figures are calculated: urban, extra-urban and a ‘combined’ figure which is a weighted average of the other two

Issues with the NEDC test

The basic test cycle was developed in the 1970s and doesn’t properly represent modern driving patterns or vehicle performance.

The cycle is only short in duration and dominated by periods of idling and low engine load

The test car only briefly reaches motorway speed

Air-conditioning, lights and other electrical loads are all switched off for the test

Flexibilities in the test procedure can be exploited to achieve a lower overall fuel consumption

The test is run in a room temperature between 20 and 30 C – much higher than the average outside temperature

The test doesn’t take account of passengers or other loads

New test from September 2018

The Worldwide Harmonised Light Vehicle Test Procedure (WLTP) replaced the NEDC as the official way of measuring fuel consumption and CO2 emissions for all new cars from 1 September 2018. It had previously been used to test new models introduced to the market between 1 September 2017 and 1 September 2018.

The aim of the new test is to produce official figures that are more realistic and more likely to be achieved in normal, real world driving, though no test can fully take account of difference in driving style or changes in traffic and weather conditions.

The new WLTP test lasts longer, covers more distance, reaches higher speeds and uses a more dynamic driving cycle that's more representative of real driving. It involves four different driving phases: low (city), medium, high and extra-high (motorway) speed and fuel consumption figures will be published for all of these alongside an overall/combined figure.

The first year VED for new cars is based on official CO2 emissions figures as is company car tax. Official CO2 emissions figures will continue to be based on NEDC test results or will be 'NEDC equivalent' calculated from the WLTP until 6 April 2020.

Toxic emissions in ‘real driving’

European regulations require toxic exhaust emissions to be effectively limited in normal driving and throughout the vehicle’s normal life rather than just in standard laboratory tests.

To achieve this a new 'Real Driving Emissions' (RDE) test is being introduced alongside the new laboratory test.

RDE uses a Portable Emissions Measurement System (PEMS) that measures NOx, CO and soot (particulates) on a vehicle while it’s driven on normal roads in a range of conditions.

The new RDE test will apply to all new models from September 2017 and to all new vehicles from September 2019.

RDE is a relatively severe test and higher emissions limits will apply compared to the laboratory WLTP test. A so called 'conformity factor' will be applied in two steps:

Step 1: RDE emissions can be up to 2.1 times (110%) higher from September 2017

Step 2: RDE emissions can be up to 1.5 times (50%) higher from January 2020 (new models) and January 2021 all new cars.

Buying a new car

If you're looking at a new car after 1 January 2019 then the quoted fuel consumption figures should be based on the WLTP and should be reasonably reliable. The figures quoted for low, medium, high and extra high speed driving will give you an idea of how fuel economy may vary depending on the type of driving you do.

If you're looking at a used car the gap will depend on the age of the vehicle – around 10% for a car first registered in the mid-2000s up to 25% or more for a car first registered in the mid-2010s.

Any single test is flawed and no single driver is representative of everyone else, but if you could see what sort of fuel economy lots of other drivers are getting in the same car that would give you a fair idea of what can be achieved taking your own driving style into account.

Real MPG (Honest John) – users have uploaded more than 100,000 fuel consumption figures. On average cars achieve 84% of the official combined fuel consumption figure but this includes many older models when the test was more representative.

Spritmonitor.de – a German site which calculates fuel economy based on users’ fuel purchase and mileage data. Fuel consumption figures are quoted in litres/100km rather than miles per gallon. (To convert litres/100km to mpg divide 62.15 by the fuel consumption in l/100km and multiply the result by 4.546)